Tuning indicator for carrier-signal receivers



Q Ll. INVENTOR JOHN C. WILSON ATTORNEY 3, 1942. J. c. WILSON TUNING INDICATOR FOR CARRIER-SIGNAL RECEIVERS Filed March 27, 1940' Feb Patented Feb. 3, 1942,

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TUNING mnrca'roa roe CARRIER-SIGNAL RECE IVERS John C. Wilson, Baysidef N. Y., assignor to Hazeltine Corporation, a corporation of Delaware Application March 27, 1940, Serial No. 326,154

'1 Claims. gen. 250-40) The present invention relates to an improved tuning indicator for carrier-signal receivers and, particularly, to a tuning indicator especially suitable for embodiment in a tunable frequencymodulated carrier-signal receiver. I

Carrier-signal receivers conventionally are provided with a manual control for tuning the receiver to a desired transmitting station. Since i the failure accurately to tune the receiver to the desired station frequently results in undesirable distortion of the received signal, it is further conventional to provide the receiver with a visual tuning indicator by which the accurate tuning of the receiver to the desired station is facilitated.

The tuning indicators of the prior art are primarily useful only with amplitude-modulated carrier-signal receivers and generally consist of some device responsive to the magnitude of a unidirectional current or voltage, for example, the magnitude of an automatic amplification control or A. V. C. potential derived from the signal energy and which, therefore, varies in accordance with the intensity of the signal translated by the receiver. The strength of the translated signal is, in general, greatest when the receiver- -is accurately tuned to the desired transmitting station and the indication of the tuning device which measures the potential derived therefrom is likewise greatest under this condition of tuning.

Carrier-signal receivers designed for highfldelity reception necessarily have broadly-tuned selector circuits which have substantially uniform response to a broad band of frequencies generally from 15 to 20 kilocycles wide. Since the majority of the prior art tuning indicators operate by virtue of the intensity of the signal energy appearing in the signal-translating channel which includes such broadly-tuned circuits. it

inregard to frequency-modulation receivers in which any difference between the resonance frequency of the sharply-tuned tuning indicator The prior art tuning indicators, in general,

have the further disadvantage that they do not give a positive indication of the direction in which the receiver is mistuned from a desired transmitting station.

I Additionally, the prior art tuning indicators, in general, cannot be embodied in frequency-modulation receivers since the intensity variations of translated signals are deliberately reduced by limiter devices, the function of which is to limit all translated signals to a substantially constant amplitude, generally that of the weakest carrier signal which the receiver is capable of reproducing satisfactorily. It is diflicult, therefore, without the use or additional apparatus, to produce in this type of receiver automatic amplification control potentials which, in responding to the intensity of received signals, can be employed as a source of'potentlal for the operation of tuning indicators.

It is an object of the invention, therefore; to provide a new and improved tuning indicator system for carrier-signal receivers which not only avoids the above-mentioned difficulties of the prior art devices but which, additionally, is of inexpensive yet sturdy construction and possesses a high degree of tuning accuracy, reliability, and sensitivity;

It is a further object of the invention to provide a tuning indicator, particularly suitable for use in frequency-modulation receivers, by which there is required an infinitesimally small operating energy to procure an indication of both the direction and degree of mistuning of the receiver from a desired carrier signal.

In accordance with one embodiment of the invention, a tuning indicator for a modulatedcarrier signal receiver including an adjustable tuning means comprises means for deriving from a received signal a control signal, the magnitude of which varies in accordance with the degree of mistuning of said receiver on either side of a received signal. The indicator includes a cathode-ray tube and means responsive to the control signal for deflecting the cathode ray of the tube to produce a visual indication of the magnitude of mistuning. There is also provided means for periodically deflecting the cathode ray in another direction to produce for each tuning position a line the position of which varies with I cults therefor comprising resistors 22,23, by-

the degree of mistuning. Predetermined portions of the visual indication are ambiguous, and

means is provided'for effectively masking these predetermined portions of the visual indication to aid in precisely tuning the receiver.

For a better understanding of the present in ventlon, together with other and further objectsv thereof, reference is had to the following descrip-' tion taken in connection with the accompanying drawing and its scope will be pointed out in the appended claims. 3

Referring nowto the drawing, Fig. 1 is a circuit diagram, partly schematic, of a complete superheterodyne frequency-modulation receiver embodying the invention in-assoclation with the balanced detector of the receiver; Figs. 2-5, inclusive,

and Fig. 8 illustrate the tuning indications provided by different operating conditions of the receiver of Fig. 1; Fig. 6 is a circultdiagram of a modification of the invention; and Fig. i illustrates atIming indication attainable by the modification of the invention of Fig. 6.

Referring more particularly to Fig. 1, there is shown schematically a complete frequency-modulation receiver of a conventional design embodying the present invention in a preferred form. In general, the receiver includes a radiofrequency amplifier I! having its input'circuitconnected to an antenna system ll,- 12 and its output circuit connected to a frequency changer or oscillator-modulator i3. Connected in cascade with the frequency changer [3, in the order named, are an intermediate-frequency amplifier l4 of one or more stages, an amplitude limiter IS, a balanced detector and tuning indicator [6, an audio-frequency amplifier H of one or more stages, and a sound reproducer l3. It will be understood that the units l0 and i3 include adjustable tuning means by which the receiver may be tuned to a desired carrier signal.

It will be understood that the various units just described, with the exception of the unit l3 comprising the present invention, may be of a conventional construction and operation, the details of which are well known in the art, rendering further description thereof unnecessary. Considering, briefly, the operation of the receiver as a whole and neglecting for the moment the operation of the balanced detector and tuning indicator IS, presently to be described, a desired frequency-modulated carrier signal is received by the antenna system H, I2, selected and amplified by the radio-frequency amplifier l0, converted to a frequency-modulated intermediate-frequency carrier signal in the frequency changer i3, amplified by the intermediate-frequency amplifier i4, limited to a predetermined substantially constant amplitude by the limiter I5, and detected by the detector of unit l8, thereby to derive the aunal a control signal the magnitude of which varies in accordance with the degree of mistuning of the receiver on either side of the received signal. This means comprises a selector network is having a band-pass characteristic sufllciently broad to pass the intermediate-frequency carrier signal and its modulation components, a pair of diode detectors 20, 2i, and individual load cirpassed by condensers 30, 3 l respectively. The selector network is comprises a. primary tuned circuit including a condenser 24 and aninductor 25,

the latter being inductively coupled to an induc 26. A radio-frequency choke 28 is connected between the junction point of the resistors 22, 23 and the center tap on the secondary inductor 26 to provide a direct current return path for the diodes 20, 2|.

. The tuning indicator provided for the receiver additionally includes a cathode-ray tube 32, here illustrated by way of example as a tube of the deflecting-electrode typ having two pairs of deflecting electrodes 33, 33 and 34, 34, arranged at right angles to each other, the first pair 33, 33 being connected across the frequency-selector or primary tuned circuit 24, 25 while the second pair 34, 34 is connected through an audio-frequency filter, comprising a series resistor 31 and a shunt condenser 33, across th detector load resistors 22, 23. A

In considering the operation of the circuit just described, it will be seen that the selector network is is, in general, similar to conventional frequency-responsive networks and, further, that the detector circuit utilizes the conventional principle of rectifying separately the sum and difference of the voltages across the primary and secondary circuits of a double-tuned selector circuit and combining diflerentially the rectified voltages so obtained to provide a voltage varying in magnitude and polarity with the frequency of a modulated-carrier signal applied thereto. That is, the circuit elements is to 3i, inclusive, comprise means for deriving from a received signal a uni directional control potential the magnitude and polarity of which vary, respectively, in accordance with the degree and direction of mistuning of the receiver on either side of the received car rier signal. The audio-frequency components of such voltage are amplified in amplifier i7 and reproduced by unit l8 in a conventional manner.

In considering the operation of the indicator, it will be assumed that the receiver is initially hadly mistuned from a desired carrier signal. "Uhder such conditions, littl or no signal appears at the output of the limiter l5 and little or no con trol signal potential is, therefore, developed across the resistors 22, 23. Consequently, no deflection potential is supplied to either of the deflecting electrodes 33, 33 or 34, 34 and the unde flected cathode ray produces a spot of light at the center of the cathode-ray screen, as indicated by the spot a in Fig. 2. As the receiver is tuned into proximity with a desired carrier signal, a control-signal potential is derived from the received signal and appears across the resistors 22, 23, the potential being larger across one of the resistors 22, 23 according to the direction of mistuning of the receiver from the received s al. I'his control-signal potential excites the defiecting electrodes 34, 34 and causes the cathode my to be deflected horizontally to one side or theother of its undefiected position a, the direction of deflection depending upon the polarity of the control-signal potential. At the same time, the

aavaces deflecting electrodes M, M are supplied with an alternating potential, the magnitude oi which varies with the degree of mistuning, thereby to deflect the cathode ray periodically in a vertical direction. There is thus formed on the cathoderay tube screen a series of lines I), c. d, e, and f as the receiver is progressively tuned toward the received signal.

In this regard, it may be noted that the oathode ray is deflected horizontally from the point a to the position of the line d as the receiver is progressively tuned toward the received signal but reverses its direction of deflection to produce the series oflines e, f, etc., as the tuning continues in the same direction. This reversal oi the horizontal deflection is produced by the gradual increase in amplitude of one of the control-signal potentials produced across the resist-' ors 22, 23, this onepotential increasing much more rapidly than the potential produced across the other resistor when the receiver is badly mistuned while the reverse condition holds true as accurate tuning is approached. The potentials across the resistors 22, 23 are of equal magnitudes and opposite polarities at the time when the intermediate-irequency signal oi the receiver has a frequency equal to the mean resonant frequency of the frequency-selector circuit Id at which time the receiver is tuned accurately to a desired carrier signal.

As the receiver is progressively tuned in the same direction through the signal, the controlsignal potential across the resistors 22, 23 reverses polarity, while its magnitude varies in a similar manner with the degree of mistuning of the receiver. There is thus produced a series of lines g, h, i. and j of progressively shorter lengths, each corresponding to a position of mistuning of the receiver to the received carrier signal. The cathode-ray beam again comes to rest at the point a when the receiver is badly mistuned on the side of the signal carrier, opposite to that iron: which the continuous tuning operation was begun. The envelope of the entire series of lines thus produced by a continuous tuning operation of this nature is symmetrical about the point a, as indicated in Fig. 2 by the broken-line envelope It.

From the foregoing explanation of the operation of the invention. it is evident that the tuning indicator produces a lin corresponding to each tuning position of the receiver on either side of the received signal carrier, the length of the line, and thus its area, and its relative position on the screen of the cathode-ray tube providing a visual indication of the magnitude and direction of mistuning of the receiver from the received carrier signal. Thus, the deflecting electrodes at, M of the tube 32 comprise means responsive to the control signal or potential, de-

rived from the received carrier signal by unit it, for deflecting the cathode ray of tube 32 to produces visual indication of the magnitude and direction or mistuning oi the receiver to the received carrier signal, and the deflecting electrodes 33, 33 and the tuned circuit 24, comprise means for periodically deflecting the oathode ray of tube 32 in another direction to produce for each tuning position a line the position of which varies with the degree of mistuning. The tuned circuit 26, 25 further comprises means responsive to the degree of mistuning for varying the magnitude of the periodic deflection in accordance with the degree of mistuning further to reference characters.

indicate by the length 01' the mistuning of the receiver.

The portions of the cathode-ray deflection iromthepointatoeitherthelinedorthelinei furnish tuningindicaticns which possibly may be misleading or ambiguous. Fig. 3 shows a modification of the invention wherein an opaque mask 39 is provided adjacent the screen of the cathoderay tube to mask from view all of the shorter lines b, c. 1. etc., no line being visible until the tuning is sufliciently accurate that the cathoderay indication lies between the lines d to i. inclusive. This arrangement thus masks the indications provided by the cathode-ray tube until the accuracy of tuning is within predetermined limits. As in Fig. 2, th line I indicates that the receiver is accurately tuned to the desired carrier signal. The mask 38 preferably is provided with lines the degree of a pair of cusps or horns A, A corresponding to the I 7 position of the indicating line for precise tuning as a further aid in indicating the condition of resonance.

The deflection patterns illustrated in Figs. -2 and 3 correspond to those obtained under the assumed condition that the primary tuned circuit 24, 28 is sharply resonant and is less than critically coupled to the secondary tuned circuit 26, 21. In the event that the primary tuned circuit is more than critically coupled to the secondary tuned circuit, the deflection pattern produced by the cathode-ray tube 32 is of the type indicated by Fig. 4. While the line I of Fig. 4 is shorter than the linese and a, by virtue of the decreased magnitude of the potential appearing across the primary tuned circuit at resonance, it still indicates the condition of exact tuning of the receiver to the desired carrier signal, since it corresponds to the position of the cathode-ray beam at zero horizontal deflection.

ing an extremely sharply-peaked response at the normal intermediate frequency of the receiver and by providing a very loose coupling between the primary and secondary tuned circuits.

The peaked characteristic of Fig. 5 may also be obtained by the modification of the invention shown in Fig. 6, wherein elements corresponding to like elements of Fig. l are designated by like The arrangement 0! Fig. Fig. 1 except that the selector network l8 has a winding 40 inductively coupled to the primary inductor 25. The potential induced in the winding 40 is supplied to the vertical deflecting electrodes 33,-33of the oathode-ray tube 32. To obtain the maximum ver tical deflection of the cathode-ray tube when the receiver is tuned to resonance with the desired carrier signal, the winding 40 is sharply tuned to the normal intermediate frequency of the receiver by a condenser 4|. The low-potential side of tuned circuit 40, M is preferably grounded as indicated. to produce a unilateral vertical deflection of the cathode-ray beam, as illustrated by the deflection pattern of Fig. 7. This has the 6 is similar to that of advantage that the length oiline f, which indicates exact tuning oi the receiver to the desired carrier signal, is greatly accentuated relative to the lines on either side corresponding to slightly detuned positions of the receiver.

While the voltage supplied to the vertical defleeting electrodes 33, 33 of the Fig. I arrangemeat is that appearing across the primary tuned circuit 24, 25, it is obvious that these deflecting electrodes might also be connected across the secondary tuned circuit 26,21. Such connection is effective to provide the deflection pattern illustrated in Fig. 8 of the drawing, the line I indieating, as inthe other deflection patterns, the point of exact tuning of the receivertothedesired carrier signal. The peculiar unsymmetrical shape of the pattern of Fig. 8 results from the iact that the voltage measured across the secondary tuned circuit is the result of the individual sum and difference voltages of the primary and secondary circuits by virtue oi the connection of the circuits through the coupling condenser 28.

It is obvious that the mask 39 of Figs. 3-5, inelusive, 1 and 8 may have other contours than V the preferred contours here illustrated without the spirit of the" invention. In !act, the mask 39 maybe omitted entirely and the same efiect obtained by deleting a similarly shaped area of the fluorescent screen of the cathode-ray tube- 32. Thus the expression effectively masking, as used in the claims, is intendeddeparting from to include any arrangement having the equivalent effect of an opaque mask as used to remove the visual efiectotherwise produced bythe cathode ray upon the fluorescent ode-ray tube.

It will also be obvious that the receiver shown screen of the oathto one skilled in the art in Fig. 1 of the drawing may be changed to receive amplitude-modulated carrier signals by simply omitting the limiter IS, the primary tuned circuit 24, 25 thereupon being connected directly to the output or the intermediate-frequency amplifier and also to a conventional amplitude detector. ther changes, the tuning indicator system shown would then operate in the manner previously described .to provide an indication oi the condition of tuning of the receiver to a desired amplitude-modulated carrier signal.

While there have been described what are at present considered to be the preferred embodiments of this inventiomit will be obvious to those skilled in the art that various changes and modi fications may be made therein without departing from the invention, and it is, therefore, aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

i. .A tuning indicator for a modulated-carrier signal receiver including adjustable tuning means comprising, means for deriving from a received signal a control signal. the magnitude of which varies in accordance with the degree of mistuning of said receiver on either side oi said received signal, a responsive to said control signal for deflecting the cathode ray of said tube to produce a visual indication of the magnitude of said mistuning, means for periodically deflecting said cathode ray in another direction to produce ior'each tuning position a line the position 01 which varies with said degree of mistuning, predetermined portions of said visual indication being ambiguous, and means for eflectively masking said predetermined portions of to aid'in precisely tuning the receiver.

2. A tuning indicator for a modulated-carrier signal receiver including adjustable tuning means comprising, means for deriving from a received signal a control signal the magnitude of which varies in accordance with the degree'oi mistun- Without furthe visual indication.

cathode-ray tube, means ing of said receiver on either side of said received signal, a cathode-ray tube, means responsive to said control signal for deflecting the oathode ray of said tube to produce a visual indication of the magnitude of said mistuning, means for periodically deflecting said cathode ray in another direction ing position a line the position of which indicates the degree of mistuning, means responsive to said degree of mistuning for varying the magnitude of said periodic deflection in accordance 'with said degree of mistuning further to indicate by the length of said lines the degree of mistuning of said receiver, predetermined portions of said visual indication being ambiguous, and means for effectively masking said predetermined portions of the visual indication to aid in precisely tuning the receiver.

' 3. Atuning indicator for a modulated-carrier signal receiver including adjustable tuning means comprising, means for deriving from a received signal a unidirectional control potential the magnitude and polarity of which vary, respectively, in accordance with the degree and direction 0! mistuning or said receiver on either side of said received signal, a cathOde ray tube, means responsive to said control potential for deflecting the cathode ray of said tube to produce a visual indication of the magnitude and means for periodi-' in another di- I direction of said mistuning,

rection to produce for each tuning position a line the position of which indicates the degree of mistuning, means responsive to said degree of mistuning for varying the magnitude of said periodicdeflection in accordance with said degree of mistuning further to indicate by the length of said lines the degree of-mistuning of said receiver,

predetermined portions of said visual indication being ambiguous,

ing said predetermined portions of the visual indication to aid in precisely tuning the receiver.

4. A tuning indicator for a frequency-modulated carrier-signal receiver including adjustable tuning means, a frequency-selector circuit, and f a frequency-detector circuit coupled thereto comprising, a cathode-ray tube having two sets 0! deflecting elements for producing deflection fields normal to each other, means for deriving from said frequency-detector circuit a unidirectional control signal the magnitude 0! which varies in accordance with the degree of mistuning of said receiver on either side of said received signal, means for utilizing said control signal for exciting a first set of said deflecting elements to produce a visual indication of the magnitude of said mistuning, means for exciting the other set at said deflecting elements from said selector circuitto vary the area of said .visual indication, predetermined portions of'said visual indication being ambiguous, and means for etfectively masking said predetermined portions of me visual indication to aid in precisely tuning the receiver.

5. A tuning indicator for a frequency-modal lated carrier-signal receiver including adjustable tuning means, a frequency-selector circuit, and

a frequency-detector circuit coupled thereto comprising, a cathode-ray tube having two sets of deflecting elements for producing deflection fields normal to each other, means for deriving from said frequency-detector circuit a unidirectionalcontrol signal the magnitude of which varies in accordance with the degree of mistuning of said receiver on either side 01 a received to produce for each tunand means for eflectively masku. -3 means responsive to said control signal for exciting a first set of said deflecting elements to produce a visual indication of the magnitude otsaid mistuning, and means for exciting another of said sets of deflecting elements from said 'selector circuit periodically to deflect said cathode ray in another direction to produce a line for each tuning position of thereceiver the length or which varies in accordance with saiddegree or mistuning, predetermined portions of said visualindication being ambiguous, and means for efl'ectively masking said predetermined portions of the'visual indication to aid in precisely tuning the receiver.

6. A tuning indicator for a modulated-carrier signal receiver including adjustable tuning means comprising, means for deriving from a.

received signal a control signal the magnitude of which varies in accordance with the degree of mistuning of said receiver on either side of said received signal, a cathode-ray tube, means responsive to said control signal 'for deflecting the cathode ray of said tube in alfirst direction in accordance with the magnitude of said mistuning, means for periodically deflecting said cathode ray in another direction to produce for each tuning position. a line deflection the position of which varies with said degree of mistuningand means for rendering predetermined portions of the said line deflections visible only within predetermined limits of tuning on either side of means for periodically deflecting said cathode ray in another direction to produce for each tuning position a line the position of which varies with said degree of mistuning, predetermined portions or said visual indication being ambiguous, and means for effectively masking portions of the cathode-ray deflection including said predetermined portions to eliminate said ambiguous indications, the contour of said masking means being proportioned to aid in selecting a predetermined one of said lines corresponding to a precisely tuned condition of said receiver.

JOHN WILSON. 

